This application claims priority under 35 U.S.C. xc2xa7xc2xa7119 and/or 365 to 00102223.5 filed in Europe on Feb. 11, 2000; the entire content of which is hereby incorporated by reference.
The present invention relates to method, system and apparatus for tone signalling with TFO (Tandem Free Operation) communication links.
In public switched telephone networks (PSTNs) tone signalling, in particular DTMF (Dual Tone Multi-Frequency) signalling, is employed as an in-band signalling method to offer the customer various services, for example control of a voice mailbox, phone banking/booking systems or other authentication procedures. By pressing one of the push buttons on a phone, a DTMF generator compiles a signal which consists of a pair out of eight different pre-defined tones. It is actually the combination of the two tones which codes a digit or letter on the pressed button. The characteristics of the transmitted tones follow a specification as described for example in xe2x80x9cCEPT T/CS 46-02 xe2x80x9cMultifrequency signalling to be used for push-button telephonesxe2x80x9d (Ref. 1).
On older analog transmission lines the characteristics of the signal could be changed for example by modulation/demodulation, level loss, added noise, or the like. As a consequence, the specification of a DTMF receiver defines the allowed deviations of a DTMF signal from its mean characteristic (see Ref. 1). In a DTMF receiver a signal is recognized as a DTMF signal when its characteristic lies within the defined limits in terms of time, level and signal-to-noise ratio (SNR).
Tone signalling, in particular DTMF signalling, can also be used in mobile communications systems, such as GSM systems. However, in the development of GSM systems it was considered that a problem would arise insofar as speech coding or speech compression is applied at a mobile telephone or mobile station MS. In such GSM systems, compressed speech is sent from the MS at 16 kbps or 8 kbps (Full Rate/Enhanced Full Rate or Half Rate). Conventionally, the speech is then transcoded and carried in the infrastructure of the system at a higher bit rate of 64 kbps (PCM). It was considered that, if generated in the MS, tone signals such as DTMF signals would, as a result of being subject to the speech coding or compression in the MS, lie outside the defined limits after transcoding to 64 kbps PCM and so would likely not be recognised by a DTMF receiver. For this reason, in GSM systems, tone signals such as DTMF signals are not generated in the MS itself but are generated on the infrastructure side, in the local mobile switching center MSC via which the MS is communicating, so that the DTMF signals are not subject to the speech coding or compression. When keys in the MS are pressed, separate out-of-band signalling messages are sent by the MS, upon reception of which a generator in the local MSC generates the DTMF signals. The DTMF signals are then sent from the local MSC in a clear, uncompressed form, at 64 kbps PCM.
This handling of DTMF is specified in the uplink (MSxe2x86x92network/infrastructure) direction in GSM systems. See xe2x80x9cSupport of Dual Tone Multi-Frequency (DTMF) via the GSM Systemxe2x80x9d, GSM 03.14, ETSI TS 100 532 V7.01, 1999-07 (Ref. 2). In the downlink direction (networkxe2x86x92telephone), however, handling of DTMF is not specified.
Generally, it was considered that specification of the handling of DTMF was not needed in the downlink direction because in most cases the DTMF signals sent from an MSC as explained above, in response to keypresses on a mobile station MS, would be intended for a destination other than a mobile station (MS), for example a voice mail service at an MSC or other services at public switched telephone network (PSTN) destinations. Therefore it was considered that the DTMF signals sent from the MSC would pass clear and uncompressed over a 64 kbps link and should not again be subjected to speech coding or compression such that they would be made unrecognisable for a DTMF receiver at the destination. Although, there is no reason in principle why DTMF signals sent from an MSC, or originating from another source such as a PSTN telephone, should not be destined for a mobile station MS, it was accepted that the DTMF signals would be subjected to speech compression or coding in the downlink to the MS and so would probably not be recognisable by a DTMF receiver at the mobile station.
In mobile communication systems, such as GSM systems, speech coding or speech compression applied as mentioned above degrades the quality of signals by introducing non-linear distortions. Generally, in Mobile-to-Mobile calls (MS-MS calls) speech coding/decoding happens at least two times, i.e. in a tandem manner. First, in the uplink direction encoding to lower bitrates (e.g. 16 kbps or 8 kbps) takes place in the first MS, with decoding/transcoding to a higher rate (64 kbps in GSM systems) in its counterpart on the infrastructure side, a first transcoder (TRA or TRAUxe2x80x94Transcoder and Rate Adaptation Unit). The second coding process happens in downlink direction, with encoding/transcoding in a second TRAU and decoding in the second MS. More coding processes could occur on long distance lines between the two TRAUs. Unfortunately, each instance of coding/decoding (codec) in such a tandem configuration adds new noise resulting in more and more degraded speech.
In order to avoid the xe2x80x9ctandemxe2x80x9d codec situation, ETSI (European Telecommunications Standards Institute) has recently specified a xe2x80x9cTandem Free Operationxe2x80x9d (TFO) protocol for Mobile-to-Mobile calls in GSM systemsxe2x80x94see xe2x80x9cInband Tandem Free Operation of Speech Codecsxe2x80x9d, GSM 08.62 (Ref. 3) and xe2x80x9cTandem Free Operation (TFO)xe2x80x9d, GSM 03.53 version 7, Release 1998; ETSI TS 101 732 V7.01 (1999-07) (Ref. 4)xe2x80x94which allows speech coding devices to be bypassed by applying inband signalling.
Using the TFO protocol, in a Mobile-to-Mobile call (MS-MS calls), speech is coded or compressed in the first MS but passes through the first and second TRAUs, in the uplink and downlink, without transcoding, to be decoded in the second MS.
The TFO protocol, in its basic operation, affects only the TRAUs and is fully compatible with existing GSM equipment. The TFO protocol is standardized for example for GSM speech traffic channels, for Enhanced Full Rate, Full Rate (16 kbps) and Half Rate (8 kbps). Using the protocol in GSM systems, the speech quality is lifted up to the high level of a single-codec configuration, once the in-band signalling protocol has verified that the prerequisites for establishing a TFO connection between two TRAUs are fulfilled.
As explained above speech signals are compressed in the MSxe2x86x92Network/Infrastructure uplink and in the Network/Infrastructurexe2x86x92MS downlink. However, proposals have been made to compress speech signals also in the infrastructure of a mobile communications network such as a GSM network, to enable a number of speech channels to be carried over a 64 kbps MSC-MSC link in the infrastructure. The equipment used for this purpose is known as Digital Circuit Multiplication Equipment or DCME. Two DCME xe2x80x9cheadsxe2x80x9d are placed between the pair of MSCs between which the MSC link is established, which heads provide complementary speech compression and decompression so that a number of channels, for example four 16 kbps channels, can be combined into one 64 kbps link between MSCs. This enables a saving of transmission costs in the inter-MSC network/infrastructure, but introduces the disadvantage that a further instance of speech compression/decompression (codec) is involved, with consequent degradation of speech quality. The speech compression used may for example be standard compression, i.e. 1:4 via EFR (Enhanced Full Rate).
The TFO protocol now offers the possibility of a cost efficient transmission within the fixed or infrastructure part of the network, but without the disadvantage of a further instance of speech compression/decompression (codec). Since a TFO connection actually requires only a reduced bitrate, for example 8/16 kbps (GSM Half Rate/Full Rate), infrastructure link, between the involved TRAUs, it is possible to carry for example four TFO channels over a 64 kbps link without the need for an additional instance of speech compression/decompression. Thus, the TFO protocol offers not only the possibility for exploiting the reduced bandwidth requirements of TFO communications but also increased speech quality, provided that DCME equipment is installed which is compatible with the TFO protocol. Such equipment is referred to as xe2x80x9cTFO Specific Circuit Multiplication Equipmentxe2x80x9d or TCME, in the context of GSM systems as xe2x80x9cGSM specific DCME equipmentxe2x80x9d or GCME.
It will also be understood that, although originally intended primarily for MS-MS calls, the TFO protocol can be applied to MS-PSTN calls when such GCME equipment is provided in a GSM system. For an MS-PSTN call, the destination GCME head transcodes from the TFO rate or compressed speech rate used between the GCME heads (e.g. 16 kbps) to for example 64 kbps for the PSTN network. Application of the TFO protocol to MS-PSTN calls can offer improvement in speech quality in that further compression, such as is always effected in current DCME multiplexing devices on long distance links, can be avoided with the TFO protocol.
Further, if calls originating from PSTN networks are subjected to speech coding/compression at the interface of the PSTN network with the mobile communications network, the TFO protocol may also be applied to such PSTN originating calls, to again offer the possibility of bandwidth savings with regard to PSTN originating calls and to avoid speech quality degradation resulting from compression/decompression in TCME/GCME equipment in the mobile communications network.
For GSM communications network operators, including DCS (Digital Communication System 1800) and PCS (Personal Communication System 1900) operators, such TFO/GCME (or combined xe2x80x9cTFO-capable TRAUxe2x80x9d and GCME) solutions would be of considerable interest, since such solutions would offer both increased speech quality and the possibility of reduced transmission costs in their core network. Hereinafter, references to GSM systems should be understood to include references also to related systems, such as DCS and PCS systems, which are largely based on GSM protocols and procedures.
However, the problem exists that the GSM TFO protocol does not support the carrying of tone signals such as DTMF signals, and GCME functionality is not standardized for the carrying of tone signalling, such as DTMF tone signalling.
The handling of DTMF signals, in a GSM mobile communications network in which it is desired to exploit the possibilities offered by the TFO protocol and GCME equipment, thus represents a problem. It is possible that this problem could be dealt with by providing that, in the event that DTMF signals are to be sent through the network, a link involved defaults back from TFO/GCME mode to conventional mode (i.e. non-TFO, non-GCME 64 kbps) to pass the DTMF signals. However, this would be expensive to implement and have significant drawbacks, not least because it would involve dynamic switching of the link between the different modes, as discussed in more detail below.
There is thus a need for a more economic, simple to implement manner of dealing with tone signals, such as DTMF tone signals in the context of TFO-capable GSM mobile communications systems and more particularly in such systems in which GCME functionality is implemented.
Problems Addressed by the Present Invention
The present invention is thus concerned with the provision of tone signalling, e.g. DTMF tone signalling, in relation to GSM TFO (tandem free operation) links, the protocol of which does not take into account the need to transport such tone signalling, employing circuit multiplying equipment (GCME, TCME).
Solution Provided by the Present Invention
According to the present invention there is provided a method as claimed in claim 1 and apparatus as claimed in claim 7.
The remaining claims set out further advantageous or preferred developments of forms of the present invention.
Advantages of the Invention
With the present invention, a method is provided for handling DTMF signalling in a GCME-GCME chain whereby system complexity is kept at a minimum and development costs are reduced. The effort and outlay needed for implementing the present invention is much less than would be involved in implementing alternative possible means of handling tone signalling in GSM TFO/GCME communications. The present invention provides for in-band handling and transportation of tone signalling information, avoiding the need to effect significant modifications to the mobile communications system.
With preferred embodiments of the invention, all inter-MSC speech traffic can be exchanged at the lower rate, e.g. 8 or 16 kbps, due to the TFO protocol in xe2x80x9cTFO operationalxe2x80x9d mode and the use of speech compression in xe2x80x9cno TFO operationalxe2x80x9d mode, and DTMF tones can also be exchanged with this lower rate of 8 or 16 kbps. There is no need to switch to a full 64 kbps duplex channel between two GCME components to handle DTMF signalling, so that dynamic switching for this purpose is avoided. The bandwidth needed for a TFO channel can be maintained and DTMF signalling can be safely passed, in-band, through this channel, whereafter the DTMF tone or tones concerned can be regenerated. Equipment for putting the invention into effect can be developed on the basis of existing network infrastructures. For example, no changes are needed with regard to the MSCs and/or the DTMF generators in the MSCs, and the handling of DTMF as specified for the MSxe2x86x92network/infrastructure uplink (see Ref. 3) need not be altered.
Moreover, preferred embodiments of the present invention can even provide for reduced transmission costs in MS-PSTN calls, since the transcoding can be moved from the TRAU handling the call from the MS to a GCME component at the edge of the PLMN (Public Land Mobile Network), where the PSTN is interfaced with the GCME component.